Provide a fallible map_entity api

## What problem does this solve or what need does it fill?

The current MapEntity trait only provides a `map_entity(entity) -> Entity` method.
There are cases where mapping the entity would fail, for example in networking. I think it would be helpful for the API to account for the possibility that the mapping fails.

Maybe we could provide a second method:
`try_map_entity(entity) -> Result`

or just modify the main api to `map_entity(Entity) -> Result`.

I think historically the api was not fallible because the only EntityMapper was SceneEntityMapper which is not fallible.
(it either maps the entity or spawns a new one)

## What solution would you like?

Update `map_entity` to return `Result`

## What alternative(s) have you considered?

Unknown